1. Field of the Invention
This invention relates to inhibition of corrosion of ferrous metals and, in particular, to inhibiton of corrosion of the cracking-type of ferrous metals in corrosive systems as contrasted with the more familiar corrosion wherein metal loss occurs and results in a thinning of the metal.
In particular, this invention relates to multifunctional corrosion inhibitors and the use of same to inhibit both general corrosion and cracking-type corrosion in corrosive systems.
Stress corrosion cracking has been defined as failure by cracking due to the combined action of corrosive material and stress, the stress being either external (applied) or internal (residual). Generally the cracking may be either intergranular or transgranular, depending upon the stressed metal and the corrosive material.
Not all metals susceptible to stress corrosion cracking are uniformly affected by any particular corrodant. For example, carbon steels are most susceptible to stress corrosion cracking in nitrate environments, copper alloys are most affected by ammonia, while austenitic stainless steels are most susceptible to stress corrosion cracking in chloride environments.
One of the troublesome areas of stress corrosion cracking has been that of austenitic stainless steels in contact with chloride environments. Some chloride solutions, such as solutions of alkaline or alkaline-earth chlorides, are so aggressive when heated that they will cause highly stressed austenitic stainless steels to crack in extremely short periods of time, which may be less than about 30 minutes. Extensively cold-worked or as-drawn parts are especially susceptible because of the high degree of internal stresses. However, even annealed parts will fail in relative short periods of time under extreme conditions and external stresses. On the other hand, completely unstressed austenitic stainless steel would be excellent for use in contact with chloride solutions because of its resistance to ordinary corrosion effects. The ferritic and martensitic stainless steels are also subject to stress corrosion cracking to a more limited extent.
Since the mechanism of stress corrosion cracking has not yet been established, the prior art has shown very little that can be done to prevent it. Some techniques have been developed, althought they are not highly successful or desirable.
Another type of stress corrosion cracking which occurs is due to the presence of hydrogen which is also called hydrogen embrittlement. This type of corrosion is due to hydrogen given off in the corrosion process and is generally aggravated by the presence of H.sub.2 S.
Hydrogen embrittlement of steel occurs when free hydrogen atoms adsorbed on the metal surface diffuse into the metal by intercrystalline or interstitial diffusion. Once in the steel the hydrogen may remain in atomic form or, upon reaching an interstitial void of larger than atomic dimensions, may combine to form internal pockets of hydrogen gas. Hydrogen is found to permeate preferentially in stressed regions and to enter the voids nearest the stressed regions.
The diffusion of hydrogen into the steel is accompanied by the formation of internal gas packets, initiation and promotion of cracks in high stress areas, and certain other phenomena which induce the condition characterized by delayed brittle failure of the steel and by reduced ability of the steel to support sustained loads.
Hydrogen embrittlement is induced in steel in a number of ways including, for example, acid pickling, cathodic cleaning, electroplating, electrochemical machining, heating in moist atmospheres, exposure to moisture under corrosive conditions as in gas and oil well drilling and production and exposure to hydrogen at elevated temperature and pressures.
Embrittlement of steels is known to occur in bodycentered cubic microstructures such as exist in tempered martensitic, bainite, lamellar pearlite and spheroidized structures, but fully austenitic steels are found to be quite resistant to such embrittlement. In general, higher strength steels, i.e., above about 200,000 p.s.i. ultimate tensile strength, are more susceptible to this type of failure although embrittlement has been found in steels having strength levels of 60,000 p.s.i. or lower. The composition of the steel is not an important factor in hydrogen embrittlement and no alloying element, either substitututional or interstitial, has been truly effective in retarding hydrogen induced delayed brittle failure.
In low tensile strength steels hydrogen absorbed in this way more frequently causes blisters rather than cracking failure.
Still another type of corrosion is corrosion fatigue which is a process of failure of alloys where alternating tensile stresses, rather than continuing tensile stresses as occurrs in stress corrosion cracking, are involved along with corrosion. There is a relationship between corrosion fatigue and stress corrosion cracking in many systems. In non-corrosion fatigue, failure starts with crack initiation at a stress riser followed by propagation due to mechanical metallurgical forces until the member fails. This propagation can occupy 90% of the specimen life. Corrosion hastens the process by causing stress rising pits to form on the surface and by causing either direct metal loss or metal weakening at the notch of the propagating crack by a stress corrosion cracking mechanism. Thus, a corrosion inhibitor effective against corrosion fatigue is both a good metal loss inhibitor as well as a good stress corrosion inhibitor.
In contrast to stress corrosion cracking, the conventional corrosion inhibitor inhibits corrosion due to metal loss by attack of the corrodant on the metal per se.
2. Prior Art
In general, amines are known to be general corrosion inhibitors. Also, amine/thiophosphate/thione reaction products are known to inhibit corrosion of the cracking-type and phosphate salts of amines are known as general corrosion inhibitors. Thus, U.S. Pat. No. 3,846,071 describes general corrosion inhibitors which are imidazoline salts, where the salt moiety is a phosphate ester of an oxyalkylated alcohol. The phosphorylating agent used to prepare the phosphate ester is orthophosphoric acid, polyphosphoric acid, acid halides of phosphoric acid and phosphorus pentoxide. Also, U.S. Pat. No. 3,959,177 describes cracking-type corrosion inhibitors which comprise a thione and a thiophosphate and which also may include an acylated amine.